1. Field of the Invention
The present invention relates to a processing equipment intended for heat treatment of workpieces, and more particularly to an apparatus for electric resistance heating and cooling tubular products.
The invention may prove most advantageous in hardening, annealing, ageing and other kinds of heat treatment of thin-walled tubular products.
2. Description of the Prior Art
Though the process of electric resistance heating metal products has been known for a comparatively long time and has found a wide commercial application, thin-walled tubular workpieces are up to now heated in vacuum, muffle and induction furnaces. This is due to the fact that existing equipment intended for electric resistance heating does not ensure maintenance of geometric shape of a thinwalled tubular workpiece during its fixing and in the process of heat treatment as well.
In particular, known in the art is an apparatus for electric resistance heating elongated products, which apparatus comprises a case with rigidly fixed brackets provided with contact clamps (see E. N. Natanzon, G. M. Telnov, Elektronagrev method elektrosoprotivleniya i elektrovysadki, Izdatelstvo "Mashinostroyeniye". Moscow, 1964, p.79). This apparatus is designed for electric upsetting with a free metal deformation. In particular, the workpiece being heated gets elongated and while being set against a fixed bracket becomes deformed and is further upset. It will be understood that fixing brackets having contact clamps rigidly to the case eliminates the possibility of applying said apparatus for heat treatment of workpieces while maintaining the shape of the latter.
Known in the art is an apparatus for electric resistance heating and cooling tubular workpieces, which apparatus provides for compensating thermal elongation of the workpiece in the process of heating and cooling thereof (see USSR Inventor's Certificate No 396377). This apparatus comprises a case provided with vertical guides, a carriage mounted on said vertical guides, a drive for vertical movement mounted on the case and connected to said carriage, a horizontal guide fixed on the carriage, two brackets provided with contact clamps mounted on said carriage so that one of said brackets is movable and is mounted on said horizontal guide. A power cylinder is mounted on the carriage with a rod thereof being pivotally connected to the movable bracket. This power cylinder is designed to put the brackets closer and to bring them apart, and to compensate for linear elongation of the workpiece (i.e. to straighten and to elongate the workpiece in the process of heating). The contact clamps are blocks adapted to envelope the tubular workpiece from outside. A bath for a cooling medium is mounted on the case under the brackets.
The apparatus above described has found application in heat treatment of various tubular products having wall thickness of more than 1 mm. However, the attempts made to apply said apparatus for heating and cooling thin-walled tubular workpieces (wall thickness ranging from 0.06 to 0.3 mm) turned out to be ineffective. Thus, in spite of careful adjustment of the apparatus, geometric shape of the workpieces could not be maintained constant. Deformation of the workpieces took place during fixation thereof in the contact clamps, in the process of heating due to the thermal expansion, and during the cooling process. It appeared that to eliminate buckling of the thin-walled workpiece, a tensile force must be applied thereto, but the value of said force is within such a narrow range that even a very fine adjustment of the power cylinder system does not ensure observance of said range. Besides, in order to transmit said force to the tubular workpiece and to ensure a good electric contact, the contact clamps must be pressed rather hard to the workpiece. However, the thin-walled tubular workpiece immediately loses stability when pressed by the blocks.